She was set to celebrate her graduation this month from Boston University: Binland Lee’s family in New York had already booked hotel rooms in Boston and her mother, brimming with pride in her only child, had purchased nearly $400 worth of senior photos — an extravagance that cost the seamstress at least a week’s paycheck.

Then on Sunday, Lee, 22, was killed when a three-alarm fire swept through her Allston apartment. Nine residents and six firefighters were injured.

The Boston Fire Department said on Thursday that the fire was caused by careless disposal of smoking materials. The Suffolk district attorney’s office is investigating to see if charges are warranted.

“We’re all heartbroken,” said Lee’s uncle, Da Ren Kwong, in a phone interview from upstate New York.